How Soils Hold Water, A Home Experiment
The short answer to how soils hold water is pores. The pores or openings between the soil particles provide space for water to reside and surface area on which water can adhere. The scientific community has expended considerable time, money and energy to understand and quantify the properties of the material called soil. However, it is possible to run a simple home-based experiment to better understand how a porous media behaves in the presence of water. Yes this is safe for children to try at home.
The porous media to be studied is the common but extraordinary paper towel. The required apparatus is one dry paper towel and a bowl containing water. First, submerge the paper towel in the bowel of water. Participants should agree that all pores are now filled with water. In the same way, a soil can have all it’s pore space filled with water, a status known as saturation. Saturation is good for forming wetlands but most agricultural crops will not grow in a soil devoid of air and tractor performance is less than desirable in this soupy condition. Next in the experiment, remove the paper towel and hold it above the bowel for three minutes. Participants will observe water draining out of the pores and should agree that drainage will stop before all of the moisture is removed from the paper towel. Similarly, the larger pores in a saturated soil will drain progressively until only the smaller pores are able to hold their water against the force of gravity, a status known as field capacity. All soils will drain to field capacity unless a restrictive layer prevents the downward movement of water. One goal of irrigated agriculture is to keep soil moisture at or below field capacity. If soil water is increased above field capacity, water will percolate through a crop’s root zone and leach agricultural fertilizer and chemicals toward groundwater.
After the paper towel has finished draining, proceed to squeeze the paper towel firmly and then squeeze a second time with all your might. Participants should note that at first a lot of water is released with little effort, then little water is released with a lot of effort and finally, there is water remaining in the paper towel that can not be squeezed out. In a similar way, plant roots exert their own tension to remove water from soil pores. Plants can extract water from soil until the remaining soil water is held too tightly by the smallest pores and as a surface film on soil particles, a status known as permanent wilting point. The range from field capacity to wilting point is known as available water or the soil water holding capacity. Even thought water is available to plants as soil moisture decreases to wilting point, the goal of irrigated agriculture is usually not to stress the plant to death. For each crop, a management allowable deficit (MAD) has been established as the percentage of available water that can be removed without unduly stressing the crop. Proper irrigation scheduling should keep soil moisture between field capacity and the MAD, a fairly small portion of the soil water continuum.
As a final note, marketing would have us believe that some paper towels will absorb more moisture than others will. This is definitely true of soils. A loamy sand will hold around 1 inch of water per foot of soil while a silt loam will hold over 2 inches per foot. If potatoes with a 1.5-foot root zone and a MAD of 35% are grown on loamy sand, the effective storage of soil water will only be around 0.5 inches. In contrast, alfalfa (4 foot root zone and 65% MAD) grown in silt loam could effectively store over 5 inches of water. The combination of soils and crops can significantly effect the soil water reservoir and the frequency of irrigation.
For soils information, County Soil Surveys and the State of Washington Irrigation Guide have been published by the National Resource Conservation Service. Also some general soil information from PNW 475, Soil Water Monitoring and Measurement" is available off the internet at Communications & Educational Support.